The invention relates to the oil lubrication of a vacuum pump, especially a rotary vane vacuum pump, and the improved construction of such a pump.
Internal combustion engines, especially those that operate according to the four-stroke cycle system, have a so-called "pressure lubrication", or "circulating forced lubrication". Here, the lubricating oil is conducted from an oil pump or oil pan through an oil pump, in particular a gear wheel pump, to the individual points of lubrication (see Dubbel's Pocket Handbook for Machine Construction, 12th Edition, Vol. II, p. 206).
Motor vehicles also frequently have a brake pressure amplifier, i.e. a power assisted braking system. Such brake pressure amplifiers contain a pneumatic piston or diaphragm system operated by a vacuum. This vacuum is preferably generated by a pump unit driven by the combustion engine of the motor vehicle. Rotary vacuum pumps have been used more and more to generate the vacuum, preferably using those constructed as rotary vane pumps.
A rotary vane pump of the type used in the present invention is known for example from U.S. Pat. No. 2,324,903 and No. 2,801,791. In these rotary vane pumps, the oil stream for sealing thr radially movable vanes with respect to the rotor and the housing is fed in under a high oil pressure and, in particular, from the end of the hollow shaft which lies opposite the drive end and which is therefore placed under less mechanical stress. The vanes of these known pumps are driven outwardly by centrifugal force and additionally by the hydraulic effect of the lubricating oil pressed into the vane base spaces. This causes a high mechanical wear on the vane tips, especially at high turning speeds, and also places an increased energy requirement on the vacuum pump, since the radially outwardly driven vanes have to be pushed back, as the rotor turns, against the oil pressure built up in the vane base spaces.
In the German Utility Pat. No. 77 07 853 (U.S. application Ser. No. 886,545, filed Mar. 14, 1978 now U.S. Pat. No. 4,231,728), a rotary vane pump has been proposed in which the lubricant feed line is arranged in the pump drive shaft to extend axially as a rigid feed line into the hollow shaft of the pump, being connected by an axially flexible coupling or bridge to the drive shaft. In this arrangement, the excess oil delivered by the oil pump of the central lubricating system runs off from the hollow shaft substantially free of pressure and is returned to the oil sump. In such an open lubricating system for the rotary pump, the oil is sucked into the vane base spaces as the vanes travel outwardly and is then displaced from the base spaces as the vanes are radially retracted into these spaces, without the occurrence of any increased power consumption. This desirable operation is aided by the fact that at high rotor velocities, the oil is foamed due to the action of the double pins of the vanes as they are moved back and forth at high speed, and the oil in this foamed state can be compressed much more easily.
One disadvantage found in the open lubricting system according to this German Utility Pat. No. 77 07 853 resides in the need to lubricate the bearings and to seal the moving vanes both in the rotor and in the casing by branching off a relatively large partial stream from the oil stream conveyed under pressure by the oil pump, i.e. using a very large excess amount of circulated oil. A limitation of the amount of oil directed to the rotary vane pump, however, would lead to rising oil temperatures and a corresponding decreasing viscosity of the lubricating oil so that the oil supply to the vacuum pump would not always be sufficient, and under unfavorable conditions, the level of the vacuum, i.e. the reduction in pressure, would become smaller and smaller. Yet another disadvantage resides in the increased power requirement placed on the gear wheel pump to handle the required amounts of lubricating oil.